Petroleum refining operations in which crude oil is processed to produce gasoline, diesel fuel, lubricants and so forth, frequently produce residual oils. The residual oil may be processed to yield valuable hydrocarbon products utilizing a delayed coker unit. When processed in a delayed coker residual oil is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields petroleum coke, a material composed mostly of carbon.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke drum, where the combined effect of retention time and temperature cause the formation of coke. Vapors from the top of the coke vessel are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. Normal operating pressures in coke drums during decoking range from twenty-five to fifty p.s.i. Additionally, the feed input temperature may vary between 800° F. and 1000° F.
The structural size and shape of coke drums vary considerably from one installation to another. However, coke drums are generally large, upright, cylindrical, metal vessels ninety to one-hundred feet in height, and twenty to thirty feet in diameter. Coke drums have a top head and a bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Coke settles out and accumulates in a vessel until it is filled, at which time the heated feed is switched to the alternate empty coke drum. While one coke drum is being filled with heated residual oil, the other vessel is being cooled and purged of coke.
Coke removal, also known as decoking, begins with a quench step in which steam, then water are introduced into the coke filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke respectively. After a coke drum has been filled, stripped and quenched, the coke is in a solid state and the temperature is reduced to a reasonable level. Quench water is then drained from the drum through piping to allow for safe unheading of the drum. The drum is then vented to atmospheric pressure when the bottom opening is unheaded, to permit removing coke. Once the unheading is complete, the coke in the drum is cut out of the drum by high pressure water jets.
Decoking is accomplished at most plants using a hydraulic system comprised of a drill stem and drill bit that direct high pressure water into the coke bed. A rotating combination drill bit, referred to as the cutting tool, is typically about twenty two inches in diameter with several nozzles, and is mounted on the lower end of a long hollow drill stem about seven inches in diameter. The drill bit is lowered into the vessel, on the drill stem, through an opening at the top of the vessel. A “bore hole” is drilled through the coke using the nozzles, which eject high pressure water at an angle approximately 66 degrees down from horizontal. This creates a pilot bore hole, about two to three feet in diameter, for the coke to fall through.
After the initial bore hole is complete, the drill bit is then mechanically switched to at least two horizontal nozzles in preparation for cutting the “cut” hole, which extends to the full drum diameter. In the cutting mode the nozzles shoot jets of water horizontally outwards, rotating slowly with the drill rod, and those jets cut the coke into pieces, which fall out the open bottom of the vessel, into a chute that directs the coke to a receiving area. The drill rod is then withdrawn out the flanged opening at the top of the vessel. Finally, the top and bottom of the vessel are closed by replacing the head units, flanges or other closure devices employed on the vessel unit. The vessel is then clean and ready for the next filling cycle with the heavy hydrocarbon feed.
After the boring hole is made, the drill stem must be removed from the coke drum and reset to the cutting mode. This takes time, is inconvenient and is potentially hazardous if the hydro-cutting system is not shut off before the drill stem is raised out of the top drum opening, operators are exposed to the high-pressure water jet and serious injuries including dismemberment occur.
In other systems the modes are automatically switched. Often, in automatic switching systems, it is difficult to determine whether or not the drill stem is in cutting or boring mode, because the entire change takes place within the drum. Mistakes in identifying whether the high pressure water is cutting or boring often occur when a cutting tool fails to switch between cutting and boring modes, which may lead to serious accidents. Thus, coke-cutting efficiency is compromised because the switching operator does not know whether or not the cutting process is complete.